Dhcp DNS Snmp

7/27/2019 Dhcp DNS Snmp

http://slidepdf.com/reader/full/dhcp-dns-snmp 1/81

www.nettech.inNettech Private Ltd.

DHCP, DNS, SNMP

Prepared by :Swapan PurkaitDirectorNettech Private [email protected]+ 91 93315 90003

For Summer Training on Computer Networking

visit www.nettech.in




First Question

• How does your computer know whichIP to use?

• How does your computer know thatwww.nettech.in is at IP 202.144.75.66?

• How can you track your network’s health?




The Internet and Addressing

• All machines on the Internet are accessed viatheir IP address – Dotted quad: xxx.xxx.xxx.xxx

• Problem: IP addresses are hard to remember and

hard to statically assign on large networks• Solution: Need to come up with a way to

automatically assign IP addresses and a textbased representation of machine addressesinstead of just 4 numbers




DHCP

• Allows client machines to receive an IPaddress, DNS information, etcautomatically

• Before DHCP came into use, users had to

type in all this information by hand, whichis bad: – Easy to mistype something when entering by

hand

– Manually changing network configuration every

time you move your laptop is a pain – Bootp resolved some of these issues

• … and DHCP still uses the same port as bootp




Purpose of DHCP

From RFC-2131 (the Internet standard):

• The Dynamic Host Configuration Protocol (DHCP) provides a framework for passingconfiguration information to hosts on a

TCP/IP network.

• DHCP consists of two components:

– a protocol for delivering host-specific configuration parameters from a DHCP server to

a host and – a mechanism for allocation of network addresses

to hosts.




DHCP functional goals

• A host without a valid IP address locatesand communicates with a DHCP server

• A DHCP server passes configurationparameters, including an IP address, tothe host

• The DHCP server may dynamicallyallocate addresses to hosts and reuseaddresses

• Hosts can detect when they require a

new IP address• Unavailability of DHCP server has

minimal effect on operation of hosts




What does DHCP do?

• Provides protocol stack, application andother configuration parameters to hosts

• Eliminates need for individual, manualconfiguration for hosts

• Includes administrative controls fornetwork administrators

• Backward compatible packet format forBOOTP interoperation (RFC 1542)

• Can coexist with hosts that have pre-

assigned IP addresses and hosts that donot participate in DHCP




Design Goals

• Eliminate manual configuration of hosts

• Prevent use of any IP address by morethan one host

• Should not require a server on every

subnet

• Allow for multiple DHCP servers

• Provide a mechanism, not a policy

• Provide the same configuration -including IP address - to a hostwhenever possible




What can you do with DHCP

• Plug-and-play

• Move desktop PCs between offices

• Renumber

• Other restructuring - change subnetmasks

• Mobile IP - laptops

• Moving equipment - cartable




What DHCP doesn’t do

• Support multiple addresses per interface• Inform running host that parameters have

changed

• Propagate new addresses to DNS

• Support inter-server communication

• Provide authenticated message delivery

• Configure routers and other network equipment

• Design network addressing plan

• Determine other configuration parameters

• Locate other servers




DHCP: Messages Overview

• Several messages are sent back and forthbetween a client and the DHCP serverbefore it can successfully obtain an IPaddress




DHCP: DISCOVER

• Hardcoding the addresses of DHCP serverskind of defeats the purpose of automaticconfiguration

• Solution: A client using DHCP will

broadcast a DISCOVER message to allcomputers on its subnet (addr255.255.255.255) to figure out the IPaddress of any DHCP servers

• Most routers are configured to pass thisrequest within the campus or enterprise




DHCP: OFFER

• (Optionally) sent from server in responseto a DISCOVER

• Contains an IP address, otherconfiguration information as well (subnet

mask, DNS servers, default gateway,search domains, etc)

• Note that all DHCP servers that receive aDISCOVER request may send an OFFER;

– since a client typically does not need morethan one IP address, more messages neededto zero on one IP




DHCP: REQUEST

• Sent by client to request a certain IP address – Usually the one sent by an OFFER, but also used to renew

leases. Also can be sent to try to get same address after areboot

• This message is broadcast

• Most OSs by default will send a REQUEST for thefirst OFFER they receive – this means that if there is a rogue DHCP server on your subnet,most clients will ignore the OFFERs from thecampus DHCP servers (since the OFFER from therogue server gets to the user’s PC first)!




DHCP: ACK/NACK

• Sent by server in response to a REQUEST

• ACK: Request accepted, client can startusing the IP it REQUESTed

• NACK: Something is wrong with theclient’s REQUEST (for example theyrequested an IP address they’re notsupposed to have)




DHCP: RELEASE

• Sent by client to end a lease

• Not strictly required, but is the “polite” thing todo if done with the IP (could just let the lease

expire)

• Some clients may not send RELEASEs in anattempt to keep the same IP address for as longas possible




DHCP: Big Picture




Summary

• DHCP works today as a tool forautomatic configuration of TCP/IP hosts

• It is an open Internet standard andinteroperable client implementations arewidely available

• Provides automation for routineconfiguration tasks, once networkarchitect has configured network andaddressing plan

• Ongoing work will extend DHCP withauthentication, DHCP-DNS interactionand inter-server communication




DHCP: Conclusion

• Efficient way of assigning computers IPaddresses and delivering configurationinformation to those computers

• Does not solve other problem of addressing – that IP addresses are hard toremember




IETF standards

• Formal process for development, reviewand acceptance of TCP/IP protocol suitestandards

• Initial specifications published as InternetDrafts (I-Ds)

• Accepted specifications published asRequest for Comments (RFCs)




Protocol status

• DHCP has been accepted as a DraftStandard; the specifications arepublished in:

– RFC 2131: Dynamic Host ConfigurationProtocol

– RFC 2132: DHCP Options and BOOTPVendor Extensions

• Several additional options are in

development




Implementation status

• DHCP is an open standard, with freelyavailable specifications

• Can be (and has been) implemented

entirely from the specification• Commercial implementations are widely

available

• Non-commerical implementations are also

available




DHCP Resources

• Compilation of DHCP-related WWW linksand other information:

http: / /www. dhcp. org

• DHCP FAQ (maintained by John Wobus)

• dhcp-v4@bucknel l . edumailing list (adminrequests to l i stserv@bucknel l . edu)

• IETF information can be retrieved from:http: / /www. i et f . cnri . reston. va. us

• I-Ds and RFCs can also be retrieved from:http: / /www. rf c-edi tor. org




DNS – Domain Name System




Second Question

• How does your computer know which IP touse?

• How does your computer know thatwww.nettech.in is at IP202.144.75.66?

• How can you track your network’s health?




Domain Name System (DNS)

• The first IP networks distributed host fileson a regular basis

– This became a burden and an automateddistributed solution was needed

• IP Addresses are great for computers – IP address includes information used for

routing.

• IP addresses are tough for humans toremember.

• IP addresses are impossible to guess.

– ever guessed at the name of a WWW site?




Hostnames

• DNS is born

• Domain names comprise a hierarchy sothat names are unique, yet easy toremember.

• The domain name system is usually usedto translate a host name into an IPaddress .

– Ex: www.xxx.org <-> IP 212.87.7.181

• (which one would you rather remember?)




DNS: Basics

• Hierarchical namespace

• Distributed system – very few coreservers

• Stores other information than simplehostname <-> IP mappings

• Request/response protocol




DNS Hierarchy

edu com org in

rpi albany ac nettech




Host name structure

• Each host name is made up of asequence of labels separated byperiods.

– Each label can be up to 63 characters

– The total name can be at most 255

characters.

• Examples:

– whitehouse.gov

– barney.the.purple.dinosaur.com

– monica.cs.rpi.edu




Domain Name

• The domain name for a host is thesequence of labels that lead from the host(leaf node in the naming tree) to the topof the worldwide naming tree.

• A domain is a subtree of the worldwidenaming tree.




Hierarchical Naming

com edu gov mil net org us

arpa

colordao

cs

foobar

foobar.cs.colorado.edu




Top level domains

• edu, gov, com, net, org, mil, …

• Countries each have a top level domain (2letter domain name).

– Such as .in, .uk, .it, etc.

• New top level domains include:

.aero .biz .coop .info .name .pro




Domain Naming System

edu com

harvard …mit

hbs ee

www

physics

cisco…yahoonasa…nsf arpa…navy acm…ieee

gov mil org net uk fr




DNS: Architecture

• DNS servers are responsible for one or moredomains of any level

• “Root servers” are maintained throughout theworld (one is in Palo Alto) and are responsible forall of the top-level domains – When you register a domain, an entry for that domain is added

to the appropriate root server

• Owners of each regular domain or subdomainmaintain (or outsource) their own DNS serverscontaining the correct information




Name Servers

• Partition hierarchy into zonesedu com

princeton …mit

cs ee

ux01 ux04

physics

cisco …yahoo nasa …nsf arpa …navy acm …ieee

gov mil org net uk fr

Rootname server

Princetonname server

Cisconame server

CSname server

EEname server

…

…

Each zone implementstwo or more nameservers

1. Primary

2. secondary




Hierarchical Administration - “Zones”

arpa com in gov mil net org us

arpa

nettech

photos photos.nettech.in




Administration - Zones

• A zone is a subtree of the DNS tree that isindependently managed – Second-level domains (“ac.in”) are usually an

independent zone

– Most sub-domains (“ximb.ac.in”) are alsoindependent.

• A zone must provide multiple nameservers. This server records the membersin the domain. – You typically need a primary name server and

one or more secondary name servers.

– Secondary retrieves information from primaryusing a zone transfer.




Resolving an address

• A.C.D wants to know about F.E.D

A

C

D

E

GB F

H

• Host “A.C.D” asks “B.C.D” (the local nameserver) to resolve “F.E.D”

Name

servers




Resolving an address

• B.C.D doesn’t know the answer.

• It wants to ask the primary domain server for the “E.D” domain, so it asks the parent of the “B.C” domain (“D” in this example) to resolve “E.D”.

• D asks H, the root server.• H doesn’t know the answer, but it’s the top-level

domain and knows that “G.E.D” is the primarydomain server for the “E.D” domain

• B.C.D now knows the primary domain server for

the E.D domain, and can now ask “G.E.D” about “F.E.D”




Domain servers

• What kind of records can berequested for a given domain? – Address translation

– Caching information

– Mail server information – Authoritative nameserver information

• How is this data requested? – Each record has a type and certain data

associated with it – clients request records of acertain type from a server




DNS Organization

• Distributed Database

– The organization that owns a domain nameis responsible for running a DNS serverthat can provide the mapping betweenhostnames within the domain to IPaddresses.

– So - some machine run by RPI isresponsible for everything within the rpi.edudomain.




rpi.edu

DNS DB

rpi.edu

DNS DB

DNS Distributed Database

• There is one primary server for adomain, and typically a number of secondary servers containing replicateddatabases.

rpi.edu

DNS DB

Authori tat ive

rpi.edu

DNS DB

Replicas

rpi.edu DNS server




DNS Clients

• A DNS client is called a resolver .

• A call to gethostbyname()is handled by a

resolver (typically part of the client).

• Most Unix workstations have the file/etc/resolv.conf that contains the

local domain and the addresses of DNSservers for that domain.




/etc/resolv.conf

domain nettech.in

202.144.75.66




nslookup

• nslookup is an interactive resolver that

allows the user to communicate directlywith a DNS server.

• nslookup is usually available on Unixworkstations. (dig and host are also DNS

clients).




DNS Servers

• Servers handle requests for their domaindirectly.

• Servers handle requests for other domainsby contacting remote DNS server(s).

• Servers cache external mappings.




Server - Server Communication

• If a server is asked to provide themapping for a host outside it’s domain(and the mapping is not in the servercache):

– The server finds a nameserver for thetarget domain.

– The server asks the nameserver to providethe host name to IP translation.

• To find the right nameserver, use DNS!




DNS Data

• DNS databases contain more than justhostname-to-address records:

– Name server records NS

– Hostname aliases CNAME

– Mail Exchangers MX

– Host Information HINFO




The Root DNS Server

• The root server needs to know theaddress of 1st (and many 2nd) leveldomain nameservers.

edu com org jp

albanyrpi




Server Operation

• If a server has no clue about where to findthe address for a hostname, ask the rootserver.

• The root server will tell you what

nameserver to contact.

• A request may get forwarded a few times.




DNS Message Format

HEADER

QUERIES

Response RESOURCE RECORDS

Response AUTHORITY RECORDS

Response ADDITIONAL INFORMATION

HEADER

QUERIES

Response RESOURCE RECORDS

Response AUTHORITY RECORDS

Response ADDITIONAL INFORMATION




DNS Message Header

• query identifier

• flags

• # of questions• # of RRs

• # of authority RRs

• # of additional RRs




Message Flags

• QR: Query=0, Response=1

• AA: Authoritative Answer

• TC: response truncated (> 512 bytes)

• RD: recursion desired• RA: recursion available

• rcode: return code




Recursion

• A request can indicate that recursion isdesired - this tells the server to find outthe answer (possibly by contacting otherservers).

• If recursion is not requested - theresponse may be a list of other nameservers to contact.




Question Format

• Name: domain name (or IP address)

• Query type (A, NS, MX, …)

• Query class (1 for IP)




Response Resource Record

• Domain Name

• Response type

• Class (IP)

• Time to live (in seconds)• Length of resource data

• Resource data




UDP & TCP

• Both UDP and TCP are used:

– TCP for transfers of entire database tosecondary servers (replication).

– UDP for lookups

– If more than 512 bytes in response - requestorresubmits request using TCP.




Lots more

• This is not a complete description !

• If interested - look at:

– RFC 1034: DNS concepts and facilities.

– RFC 1035: DNS implementation and protocol

specification.

– play with nslookup.

– Look at code for BIND (DNS server code).




Name to Address Conversion

• There is a library of functions that act asDNS client (resolver).

– you don’t need to write DNS client code to useDNS!

• With some OSs you need to explicitly linkwith the DNS resolver library:-lnsl (nsl is “Name Server Library”)

Suns (Solaris) need this !




DNS library functions

gethostbyname

gethostbyaddr

gethostbyname2




gethostbyname

struct hostent *gethostbyname( const

char *hostname);

struct hostent is defined in netdb.h:

#include <netdb.h>




struct hostent

struct hostent {

char *h_name;

char **h_aliases;

int h_addrtype;int h_length;

char **h_addr_list;

};

official name (canonical)

other names

AF_INET or AF_INET6

address length (4 or 16)

array of ptrs to addresses




Which Address?

On success, gethostbyname returns theaddress of a hostent that has beencreated.

– has an array of ptrs to IP addresses

– Usually use the first one:

#define h_addr h_addr_list[0]




gethostbyname and errors

• On error gethostbyname return null.

• Gethostbyname sets the global variable h_errno

to indicate the exact error: – HOST_NOT_FOUND

– TRY_AGAIN

– NO_RECOVERY

– NO_DATA

– NO_ADDRESS All defined in netdb.h

Getting at the address:




Getting at the address:char **h_addr_list;

h = gethostbyname("joe.com");

sockaddr.sin_addr.s_addr =

*(h->h_addr_list[0]);

This won't work!!!!

h_addr_list[0] is a char* !




Using memcpy

• You can copy the 4 bytes (IPv4) directly:

h = gethostbyname("joe.com");

memcpy(&sockaddr.sin_addr,

h->h_addr_list[0],

sizeof(struct in_addr));




Network Byte Order

• All the IP addresses returned via thehostent are in network byte order!

• Repeat after me:

"Thank you gethostbyname!"




gethostbyaddr

struct hostent *gethostbyaddr( const

char *addr

size_t len,

int family);




Some other functions

uname : get hostname of local host

getservbyname : get port number for a

named service

getservbyaddr : get name for service

associated with a port number




References

• RFC 1034

• RFC 1035




SNMP - Simple Network ManagementProtocol




Third Question

• How does your computer know which IP touse?

• How does your computer know thatwww.nettech.in is at IP 202.144.75.66

• How can you track your network’shealth?

SNMP i




SNMP is …

• A network management tool

– Designed to allow data acquisition from SNMPcapable network equipment

– Allows control of SNMP capable equipment by

the distant controller• Based on inquiry/response architecture

with simple response parameters

SNMP is




• Used for management of fault conditions –prevention of and response to networkfailure

• Designed to manage performance –

coping with deterioration of performancedue to congestion, changing trafficconditions

SNMP is …

SNMP M P t l




SNMP Message Protocol

• Messages use UDP, ports 161 (requests/responses) and162 (notifications)

• Message types:

– GetRequest: request values of variablesfrom device

– GetNextRequest: request value of variablefollowing the one supplied

– GetResponse: return values

– SetRequest: instruct device to set values of variables

– Trap: from device - notify monitor /manager of value change




To know more on SNMP

• “Essential SNMP”, Douglas R Mauro & Kevin J Schmidt, Shroff Publishers & Distributors, Calcutta, 2001



Connect with us at Facebook

Visit www.facebook.com/nettech.in

Dhcp DNS Snmp

Documents

Transcript of Dhcp DNS Snmp